1) Field of the Invention
The present invention relates to a safety valve using a soluble plug, more particularly, relates to a safety valve for use in a fuel tank of automobiles which are powered with combustible gas as a fuel.
2) Related Art Statement
For a fuel tank mounted in automobiles which uses combustible gas, such as natural gas, as a fuel for engines, often used a plastic tank in order to make the weight lighter. A tank made of plastic is light in weight, however, does not have sufficient strength against an increase of heat or pressure, compared to a metal tank.
Further, automobiles have factors in increasing the temperature of the body itself due to heat from an engine, or sunshine, and therefore, the temperature in the place where the fuel tank is arranged becomes remarkably higher than the temperature outside of the body.
Therefore, a safety valve is used in fuel tanks of automobiles in order to prevent the explosion of a tank caused by an increase of inner pressure which accompanies an increase of temperature of a tank. This safety valve is arranged to melt when the temperature in the atmosphere around the valve increases and the valve is open.
FIGS. 1 and 2 are cross sectional views showing a construction of a conventional safety valve for use in fuel tanks of automobiles. The safety valve 9 shown in FIGS. 1 and 2 comprises a valve body 91 and a path 92, which is formed through the body 91 for exhausting a gas, and a soluble material 93, which is filled in the path 92 as a soluble plug.
The conventional safety valve 9 functions in such a manner that when the temperature around the valve 9 increases to a predetermined value, the soluble material 93 melts; the melted material 93 is then pushed out to the outside of the path 92 by the pressure of a gas in the tank; the path 92 is connected to the outside of the valve 9, and the gas is then exhausted through the valve 9.
According to the construction of the conventional safety valve, however, the gas pressure is constantly applied to the soluble material 93. Therefore, the soluble material 93 of the soluble plug causes a creep and is exhausted out from the valve body 91 over a long period of time. Further, a creep is apt to be generated when the temperature of the soluble plug 93 increases.
The safety valve, where the soluble material 93 is exhausted as shown in FIG. 5, cannot realize an initially designed function thereof any more.
It is considered to make the cross sectional area of the path 92 smaller in order to make the area of the plug against which the gas pressure is applied smaller for restricting the generation of creep. However, this construction also makes the cross sectional area of an exit path for exhausting a gas small, so that a sufficient area of the gas flow path, which is necessary to exhaust the gas, cannot be secured. On the other hand, in order to assure the sufficient area of the path for exhausting the gas, the dimension of the safety valve itself should be made large.
Automobiles having a combustion engine have many factors in that the temperature of the fuel tank increases.
For instance, under the circumstance that an atmospheric temperature is 40xc2x0 C., first, the temperature of the surface of the fuel tank increases to about 52xc2x0 C. under direct sunshine: second, the temperature further increase by 5 to 10xc2x0 C. due to the by-product heat of the engine: third, in case the automobiles use natural gas as a fuel, when the fuel is filled into the fuel tank, the temperature in the tank increases. In such a case, for instance, the fuel is normally filled into the tank until the inner pressure of the tank increases from 20 atmosphere to about 200 atmosphere in a short period (3-5 minutes), when the gas is charged in the tank; then the gas charge increases the temperature inside the tank by about 15xc2x0 C. due to adiabatic compression.
If all of the factors occur at once, the temperature in the tank, becomes 72-78xc2x0 C.; in addition to this, abut 200 atmospheric pressure is applied against the safety valve. If such a condition is maintained that a high pressure is applied and the temperature of the tank becomes so high, the conventional safety valve becomes likely to cause more creep.
The present invention has for its purpose to provide a safety valve in which the leak of a soluble plug due to creep is restricted and can be used for a long term. In order to carry out this purpose, the safety valve according to the invention, which is attached to a gas flow path in a gas container mounted in a movable body or in the vicinity of the container, comprises a valve body having an opening which opens to a gas atmosphere, a cylinder portion being formed in said valve body and being connected to said opening, a piston being contained in said cylinder portion in a freely slidable manner, a containing portion being filled with a soluble material, which is melted at a predetermined temperature or higher, and being constructed such that a capacity thereof being reduced by moving of said piston, a connecting path connecting said containing portion and an outside of said valve body, an energizing means being inserted between said piston and said valve body for energizing said piston in a direction for that the capacity of said containing portion is reduced, an exhausting path connecting said cylinder portion and said outside of the valve body and becoming in an open condition when said piston moves in a direction for that the capacity of the containing portion is reduced, an air space being formed between a closed end portion of said cylinder and said piston, a first gas pressure receiving surface being formed at one of end portions of said piston and receiving a gas pressure from said container side and a second gas pressure receiving surface being formed at another one of end portions of said piston and receiving a gas pressure from said air space side, and a path connecting said airspace and said cylinder portion which exists at an opening side with respect to said piston.
The present invention has an aspect in that said path is formed inside of said piston and opens to said first and second gas pressure receiving surfaces, respectively.
The present invention has another aspect in that said first and second gas pressure receiving surfaces have the same area.
Further, the present invention has still another aspect in that said energizing means has an energizing force more than a frictional resistance between said piston and said cylinder portion and necessary to push the melted soluble material out.